This invention relates to a vehicle coupling in the form of a weight transfer hitch for releasably connecting two vehicles such as a tractor and its trailer.
Although the agricultural tractor was developed mainly as a tool for cultivation, it is unrivalled as an efficient and economical prime mover, provided that a large proportion of its available power can be converted to tractive effort. This tractive effort can be expressed either as "available" rimpull or "usable" rimpull. "Available" rimpull is dependent upon the horsepower of the tractor engine, and "usable" rimpull (which is the important factor in determining the load which can be pulled by the tractor) is the product of the weight on the driving wheels of the tractor and the coefficient of friction between the driving wheels and the surface of the ground. With the use of rubber tyres, the coefficient of friction varies between about 0.9 on dry concrete to about 0.12 on ice. In the worst operating conditions in, for example, a cane field, the coefficient of friction may be expected to range between 0.3 and 0.4. Thus, an agricultural tractor having a rear axle weight of 3,500 lbs., operating on a muddy surface, may be expected to generate "usable" rimpull in the range of 1,050 to 1,400 lbs., which would be bearly sufficient to overcome the rolling resistance of a vehicle with a gross weight of only 5 tons.
An obvious solution to the shortage of "usable" rimpull is to utilise the payload to increase the weight on the driving wheels, and almost every single-axle agricultural trailer employs this principle. In most cases, however, the improvement is limited, since the weight is transferred to a point behind the centre line of the rear axle of the tractor, tending to cause the front wheels to lift clear off the ground.
Weight transfer couplings have been developed to transfer such weights within the tractor wheel base, with spectacular improvements in "usable" rimpull. For example, the "usable" rimpull developed by the tractor described above would be increased to between 3,750 and 5,000 lbs., permitting a payload of up to 10 tons in the very poor conditions described above. Such a weight transfer coupling usually includes three mutually perpendicular pivots so as to accommodate relative pitching, rolling and yawing movements between a tractor and its trailer. Unfortunately, known weight transfer couplings suffer from stability disadvantages, particularly during cornering. These stability problems arise from the natural constraints relating to the positioning of the three axes. Thus, the humping axis must be within the wheelbase of the tractor, and the yawing pivot must be connected to the trailer. However, to increase stability, the yawing axis must be as near as possible to the humping axis, and known couplings have been unsatisfactory in this respect mainly because of the positioning of the rolling pivot and its support means.